Spark plugs generally include a metal shell, an insulator, a center electrode, and a ground electrode.
The metal shell has a threaded portion on outer periphery for fitting the spark plug into a combustion chamber of an engine.
The insulator has a bore formed therein. The insulator is fixed in the metal shell so that a first end thereof protrudes from a first end of the metal shell and a second end thereof protrudes from a second end of the metal shell.
The center electrode is secured in the bore so that a first end thereof protrudes from the first end of the insulator.
The ground electrode is joined to the metal shell. The ground electrode is opposed to the center electrode through a spark gap therebetween.
A terminal is partially inserted in the insulator bore and is fixed on the second end of the insulator. A first end of the terminal is connected to a second end of the center electrode in the bore. A second end of the terminal protrudes from the second end of the insulator so that an ignition coil can connect to the second end of the terminal.
The above mentioned spark plug is inserted into a plug hole of an engine and is fixed to the plug hole with the threaded portion of the metal shell so that the spark gap is disposed in a combustion chamber of the engine.
In recent years, since the engine has been downsized for facilitating installation, it has become more popular to insert the ignition coil, which connects to the spark plug for providing high voltage, into the plug hole.
The terminal and the insulator protruded from the metal shell are inserted into the plug cap of the ignition coil, so that the ignition coil is connected to the spark plug in the plug hole. The ignition coil provides high voltage to the center electrode through the terminal.
It is important to facilitate the attachment of the plug cap to the spark plug. However, the attachment of the plug cap to the spark plug in the plug hole is often not smooth, because the plug cap is attached to the spark plug deep in the plug hole, which is itself narrow.
A spark plug having a small diameter insulator is proposed in Japan unexamined utility model publication No. H5-55489. However, that spark plug does not effectively facilitate the attachment.
Furthermore, there is a large diameter difference between the terminal and the second end side of the insulator, a step portion formed by the diameter difference prevents the plug cap from attaching smoothly. Simply enlarging an inner diameter of the plug cap facilitates the attachment of the plug cap. However, if the contact between the spark plug and the plug cap is insufficient, spark discharge will occur at the second end side of the insulator. Such spark discharge is called flashover. It is an undesirable phenomenon in which spark discharge occurs between the metal shell and the terminal through the clearance formed by the spark plug and the plug cap.